CN-0217 Datasheet, PDF (4/6 Pages) Analog Devices – High Accuracy Impedance Measurements Using 12-Bit Impedance Converters

CN-0217
Example 2: kΩ Impedance Range
Using an RCAL of 99.85 kΩ, a wide range of unknown impedances
were measured according to the setup conditions listed in Table 4.
Figure 6 to Figure 10 document accuracy results. To improve
the overall accuracy, select an RCAL value closer to the unknown
impedance. For example, in Figure 9, an RCAL closer to the ZC
value of 217.5 kΩ is required. If the unknown impedance range
is large, use more than one RCAL resistor.
Table 4. kΩ Impedance Range Setup for VDD = 3.3 V Supply
Voltage
Parameter
Value
Voltage Peak-to-Peak (V p-p)
0.198 V (Range 4)
Number of Settling Time Cycles 15
MCLK
16 MHz
RCAL
99.85 kΩ
RFB
100 kΩ
Excitation Frequency Range
30 kHz to 50 kHz
Unknown Impedances
R0 = 99.85 kΩ, R1 = 29.88 kΩ,
R2 = 14.95 kΩ, R3 = 8.21 kΩ,
R4 = 217.25 kΩ, C5 = 150 pF,
(ZC = 26.5 kΩ at 40 kHz),
C6 = 47 pF (ZC = 84.6 kΩ at
40 kHz)
120
110
100
IDEAL
90
80
MEASURED
70
60
30
35
40
45
50
FREQUENCY (kHz)
Figure 6. Magnitude Result for ZC = 47 pF, RCAL = 99.85 kΩ
Circuit Note
–89.3
–89.4
–89.5
–89.6
–89.7
–89.8
–89.9
–90.0
–90.1
–90.2
–90.3
30
35
40
45
50
FREQUENCY (kHz)
Figure 7. Phase Result for ZC = 47 pF, RCAL = 99.85 kΩ
8280
R3
8260
8240
8220
8200
8180
IDEAL
8160
30
35
40
45
50
FREQUENCY (kHz)
Figure 8. ZC = 8.21 kΩ, RCAL = 99.85 kΩ
218.5
218.0
217.5
IDEAL
217.0
216.5
216.0
R4
215.5
21.50
214.5
214.0
213.5
30
35
40
45
50
FREQUENCY (kHz)
Figure 9. ZC = 217.25 kΩ, RCAL = 99.85 kΩ
Rev. A | Page 4 of 6